1. Field of the Invention
The present invention relates to a fuel supply and injection system in a vehicle for supplying fuel to fuel injection valves from a fuel tank by an electric pump unit and injecting the fuel into combustion chambers of an engine main body of an engine through the fuel injection valves. The present invention also relates to a method for controlling such a system.
2. Description of Related Art
A spark ignition internal combustion engine (generally known as a gasoline direct injection engine and hereinafter referred to as direct injection engine) has been known as one type of internal combustion engine for a vehicle. As shown in FIG. 15, the direct injection engine has a plurality of fuel injection valves 102 for directly injecting fuel into respective combustion chambers (cylinder bores) 101 of an engine main body 100. The direct injection engine is advantageous over a conventional type of engine, in which fuel is injected into an intake pipe, for the following reasons. That is, since the fuel is not injected into the intake pipe, the direct injection engine shows quicker response. Also, due to cooling effect exerted in each cylinder, the amount of intake air is increased to increase the engine power, and abnormal combustion, such as knocking, is less likely to occur.
Fuel within a fuel tank 105 is supplied to the respective fuel injection valves 102 by two fuel pumps 106, 108 and two (first and second) fuel lines 110, 111. Of the two fuel pumps, the fuel pump 106 is an electric pump (generally known as a feed pump), which is provided inside the fuel tank 105 and generates a relatively low pump pressure. The other fuel pump 108 is a mechanical pump, which is provided to the engine main body and generates a relatively high pump pressure. The electric pump 106 and the mechanical pump 108 are connected to each other through the first fuel line 110.
More specifically, the electric pump 106 pumps the fuel from the fuel tank 105 to the first fuel line 110 at a relatively low pressure (from 0.3 to 0.5 MPa). The fuel pressure between the electric pump 106 and the mechanical pump 108 is relatively low, so that the first fuel line 110, which connects between the two pumps 106, 108, does not have to withstand a relatively high pressure, and therefore, can be made of rubber. The mechanical pump 108 is driven by rotation of the engine and pressurizes the fuel, which has been pumped by the electric pump 106 at the relatively low pressure, to a relatively high pressure (from 5 to 14 MPa) to pump the pressurized fuel to the second fuel line 111. A metal pipe is used to form the second fuel line 111 because it has to withstand the relatively high pressure. The fuel pressurized by the mechanical pump 108 is branched by a delivery line 112 and is supplied to the respective fuel injection valves 102 provided to the corresponding combustion chambers (cylinders).
Opening and closing of each fuel injection valve 102 is controlled by an ECU 115. The ECU 115 computes an amount of fuel (fuel injection amount) that needs to be injected based on a fuel pressure value measured with a fuel pressure sensor 116, an intake pressure sensor, an air-fuel ratio sensor (the latter two are not shown), etc. Accordingly, the pressurized fuel is injected into the combustion chambers 101 through their respective fuel injection valves 102 based on the fuel injection amount thus computed.
The previously proposed fuel supply and injection system for the direct injection engine uses the electric pump 106 and the mechanical pump 108 as the fuel pumps and uses the relatively high pressure (from 5 to 14 MPa) as the injection pressure.
The above arrangement, however, poses the following disadvantages. Firstly, the use of the electric pump 106 and the mechanical pump 108 results in the relatively complicated structure, which increases the manufacturing and assembling costs. Secondly, the use of the relatively high injection pressure requires the relatively large mechanical pump 108 and a greater driving force for driving the mechanical pump 108 to achieve the relatively high pressure. Thirdly, it is difficult to maintain a required fuel injection amount and a required level of fuel atomization at the engine start-up.
More specifically with respect to the third disadvantage, the mechanical pump 108 cannot pressurize the fuel as high as 5 to 14 MPa while the engine speed is relatively low at the engine start-up (the fuel is pressurized to 0.2 to 0.5 MPa at most by the electric pump 106), and only the fuel at the relatively low pressure is supplied to the fuel injection valves 102. On the other hand, each fuel injection valve 102 is designed to ensure a required level of fuel atomization and a required fuel injection amount only at the desired predetermined fuel pressure (from 5 to 14 MPa). Hence, when the fuel pressure is lower than the predetermined pressure, that is, when the fuel pressure is in a range of (0.2 to 0.5 MPa)/(5 to 14 MPa)=1/10 to 1/70, atomization of the fuel injected into the corresponding cylinder becomes insufficient, and also a fuel injection amount becomes insufficient. This causes the following disadvantages. That is, a level of combustion of the fuel at the engine start-up is lowered. Also, time required for the engine start-up is lengthened. Furthermore, emissions of harmful gas, such as smoke and HC, are increased.